eo and µo describe the electrical
and magnetic characteristics of a vacuum -- or free space.

For vacuum, their values give a speed of 3.00 x 108
m/s for the electromagnetic wave. This is so close to the measured
speed of light that it could not be considered a mere coincidence.
Further investigation showed light to be an electromagnetic wave;
light is a moving disturbance of electric and magnetic fields.

EM waves, like all others, may be described in terms of
wavelength or in terms of frequency. Radio waves and infra-red
radiation are EM waves with wavelengths longer than light. X-rays
and ultra-violet radiation are EM waves with shorter wavelengths.
EM waves with still shorter wavelengths are known as gamma rays.
The figure below illustrates the wide range of wavelengths or
frequencies available in EM waves. Such a description is called
the EM spectrum. All are electromagnetic waves. Due to the
enormous variation in wavelength, waves with different labels are
produced and detected by very different means.